The invention relates to a control apparatus for a hydraulic motor, comprising a two-directional control valve connectable by a supply conduit to a pump connection, by a discharge conduit to a container connection and by two motor conduits to the motor, a compensating valve in the supply conduit that holds the upstream pressure drop at the control valve substantially constant, and a load pressure conduit which is for influencing the compensating valve and possible the pump pressure and has two starting sections each for joining to a respective load pressure sensing point in the control valve operative depending on the direction as well as a throttling point, a branch conduit and an over-pressure valve therebehind leading to the discharge conduit.
Such control apparatuses are, for example, known from DE-OS 33 02 000 U.S. Pat. No. 4,548,259. The control valve and compensating valve together form a proportional valve in which the position of the control valve slide corresponds to a particular motor speed. The load pressure is tapped at the outlet of the control valve; it, together with a spring, balances the slide of the compensating valve in the opening direction whereas the inlet pressure of the control valve balances it in the closing direction.
If the load pressure exceeds a predetermined value, the over-pressure valve will respond. The discharged pressure medium will cause a pressure drop at the throttle. The opening load of the slide of the compensating valve becomes less. The compensating valve moves in the closing direction. The pressure is limited to the opening pressure of the over-pressure valve. In contrast with pressure relief valves which connect the motor conduit to the container, one has the advantage that only comparatively small amounts of liquid have to be led off. This reduces energy losses and permits one to operate with smaller pumps.
DE-OS 26 56 059 U.S. Pat. No. 3,987,623 discloses a similar control apparatus in which a plurality of blocks, each for one motor, contain a two-directional control valve, a compensating valve and an over-pressure valve and are supplied by a common pump. In this case, the load pressure conduit is directly connected to the two motor conduits by a change-over valve. In each block, the pressure is individually regulated by the compensating valve by the load pressure whereas the pump pressure is influenced by the respective highest load pressure.